The argument for a hypersonic missile testing ban

China and the United States recently conducted development tests of hypersonic missiles, designed to travel many times the speed of sound and reach targets thousands of miles away in an hour or less. Both tests failed, showing that hypersonic propulsion and flight—which involve extreme airspeeds, temperatures, pressures, and stresses, along with compact airframes and low weight—remain unproven technologies, even after decades of development. Solutions to the technological challenges of hypersonic flight may soon be within reach, using new materials and high-performance computing. But it is inconceivable that hypersonic weapons could be perfected and validated for operational use without extensive additional testing programs.

For this reason, it is important for the United States—the world leader in hypersonic missile development—to pursue an international hypersonic missile test ban, which could be easily verified and, once put in place, would end an arms race that already involves the United States, China, Russia, and India, with France, the United Kingdom and probably other countries lurking in the wings.  

What are hypersonic missiles? Hypersonic missiles fall into two distinct categories. In what is known as a boost-glide weapon, the hypersonic vehicle is first “boosted” on a ballistic trajectory, using a conventional rocket. It may cover considerable distance as it flies to high altitude, then falls back to Earth, gaining speed and finally, at some relatively low altitude, pulling into unpowered, aerodynamic, horizontal flight. After that, it glides at hypersonic speed toward its final destination.

Hypersonic cruise missiles, on the other hand, typically are launched to high speed using a small rocket, and then, after dropping the rocket, are powered by a supersonic combustion ram jet, or scramjet, for flight at five times the speed of sound (some 3,800 miles per hour) or greater.

The recent failed Chinese and American tests were of boost-glide systems; the X-51 WaveRider, which the US successfully tested last year after a string of failures, is an example of the scramjet cruise missile. The boost-glide test failures were probably caused by issues with booster rockets rather than with the hypersonic gliders themselves. Regardless, these systems didn't work, demonstrating that both boost-glide and powered hypersonic cruise missiles require testing. Such tests are easily observable from space and via radar and signals intelligence gathering (not to mention old-fashioned human spying). A test moratorium would throw a huge obstacle in the path of hypersonic programs, and a permanent test ban would make it clear that they aren’t going anywhere. And that would be a good thing, because where they are going is nowhere good.

In the US, hypersonic missiles have been billed as a method for quickly delivering conventional warheads when time is of the essence; one example often given is attacking a terrorist stronghold promptly when intelligence indicates the opportunity to kill a high-value target. In practice, however, the most successful attacks in counter-terrorism operations have been launched not from thousands of miles away but from nearby, using ground forces, manned aircraft, or drones.

Some proponents are more open in contending that hypersonic weapons should be developed to provide a capability for attacking strategic military targets within the territory of a major military power, including, as defense consultant Timothy A. Walton suggests, “space downlink systems, fixed anti-satellite tracking and firing systems, key radars and associated systems, command and control nodes, long-range surveillance systems, and possibly even transporter erector launchers (TELs), TEL reload points, or ships in port.” Proponents claim that the use of hypersonic weapons with conventional warheads (or pure kinetic energy) would permit such attacks to be carried out on the home territory of a nuclear power without the risk of a nuclear response. But Chinese and Russian officials have indicated they fear that US hypersonic weapons could be used to lead a nuclear first strike, and both those countries have begun hypersonic missile programs of their own, suggesting that, absent controls, a full-scale hypersonic arms race is just around the corner.

It’s not often that one can say an entire technology should be banned because it has no conceivable good use. Hypersonic missiles, however, may present just such a case. Hypersonic air travel seems economically unjustifiable in an era of climate change, high-cost energy, and low-cost video telepresence. But if civilian hypersonic air travel ever did become a reality, it would take the form of a large airplane, not a small missile. Low-cost satellite launches? Hypersonic space planes such as DARPA’s planned XS-1, which would lift rockets to high altitude, might make some sense. But again, to achieve economies of scale, such hypersonic boosters would need to be large, which the hypersonic missiles under military development are not.

Because hypersonic missiles now in development have no foreseeable civilian role and no likely military role outside of major war between nuclear-armed states, and because a hypersonic arms race would contribute to strategic destabilization, it would seem reasonable to seek a way to avert such a race. A ban on hypersonic missile testing could do just that with, most likely, little need for extensive negotiations on verification.   

How the hypersonic race started. In the days after the 9/11 terror attacks on New York and Washington, hypersonic weapons were sold as a form of “conventional prompt global strike,” fulfilling the supposed need for a weapon that could be launched from fortress America and strike Osama bin Laden’s lair on the other side of the world in less than an hour. That was an idea so unlikely that, unfortunately, few outside policy circles took the matter seriously, even when it was later revealed that another aim of prompt global strike was to destroy Chinese anti-satellite weapons before they could be launched. It was never very clear why China would be less offended by our targeting their weapons and facilities deep inside the Chinese mainland with hypersonic cruise missiles rather than somewhat faster ballistic missiles. The vague reasoning is that the different flight profile of these weapons would distinguish them from nuclear ballistic missiles, and the Chinese (or Russians, in some other scenarios) would trust that American hypersonic missiles do not carry nuclear warheads. But US non-nuclear hypersonic weapons could well be targeting nuclear weapons and supporting facilities, possibly in preparation for a follow-on nuclear strike. Furthermore, there is no physical reason why hypersonic missiles could not deliver nuclear warheads.

Although these weapons are slower than ballistic missiles, they are still very fast and offer an attack profile that presents a qualitatively new threat to adversaries. The idea that they might be used to attack a nuclear power, because they would be easily distinguished from ballistic missiles and the enemy would be willing to believe that no hypersonic weapon carried a nuclear warhead, can hardly be considered stabilizing. Rather, it is a challenge to nuclear deterrence and a threat that potential adversaries have to take seriously. They will respond by seeking countermeasures, including symmetrical capabilities, as China and Russia are doing, via their own hypersonic missile programs. To imagine that a strategic attack could be carried out on military forces based in the homeland of a major nuclear power without the risk of a nuclear response is a dangerous fantasy. But the race to develop and deploy hypersonic weapons is serious.

Hypersonic missiles are not intended only for deep land attack; they are also likely to be used at sea, for attacking ships, island bases, and shore facilities. BrahMos, a Russian-Indian joint venture, has already produced the world’s fastest supersonic cruise missile, and that venture is working on a hypersonic version, with the anti-ship role as a major aim. Shortening the strike time for naval missile warfare is a recipe for hair-trigger confrontation between major powers contending for regional or global dominance. If there is a way to stop or slow this development, the world should take it.

How to stop the hypersonic missile race in its tracks. Fortunately, a hypersonic missile test ban would be one of the most rigorously verifiable arms control measures one could imagine. It could begin with an informal moratorium, agreed and announced among the major players, and followed up by negotiations for a binding, permanent ban treaty.

To speed its approval, any such moratorium would have to define hypersonic missiles in a way that does not require elimination of already existing cruise missile systems. I would propose a ban on flights of any aerodynamic vehicle of less than, say, 15 meters length or 2 meters diameter, traveling in powered or unpowered flight at speeds in excess of 1 kilometer per second, over a horizontal distance greater than 100 kilometers. Space and ballistic missile launches and reentries could be specifically excepted. The numbers are somewhat arbitrary and could be fine-tuned or adjusted substantially while preserving the intent of the agreement. The numbers suggested here would permit Russia and India to retain their BrahMos 1 supersonic cruise missile, while forcing them to cancel the hypersonic BrahMos 2. The United States and China would then be permitted to develop supersonic systems comparable to BrahMos 1 but would have to cancel their hypersonic programs. While an even lower speed limit would be desirable, canceling future programs verifiably via a test ban should be an easier sell to other countries than eliminating existing, proven, deployed systems.

The United States should take the lead in proposing a hypersonic missile test moratorium and seeking a permanent hypersonic test ban. Of course, production, deployment, and use of hypersonic missiles should eventually be prohibited under a permanent treaty. But a test ban is the critical element; it would be reliably verifiable, and it would all but preclude production and deployment. Nations do not generally go to war relying on untested weapons, and hypersonics are a technology particularly in need of thorough testing both to perfect and to validate weapon systems. Tests of hypersonic missiles are easily identified by their flight profiles, speeds, and the heat and turbulence generated by hypersonic flight. US tests are conducted at sea and could be observed by China and Russia from international waters; Chinese and Russian tests over land could be observed by the US from space and, to some extent, from friendly countries.

Hypersonic missiles are a new class of weapon that no country actually needs. Their military advantages are ill-defined, and their capacity to destabilize relations among major powers and contribute to a costly and dangerous strategic arms race is enormous. Even so, the United States can’t expect that just because it proposes a test ban, other nations will line up to renounce hypersonic missiles. What America can reasonably hope is that other nations will see their shared interest in avoiding or slowing a dangerous escalation of the arms race. I am therefore proposing that the United States suspend testing for a while, to show good faith as it seeks agreement on an international ban on hypersonic testing.

If the suspension does not draw a positive response from other countries, the United States can always resume its programs, while still advocating a general moratorium, thereby seizing the moral high ground. That others might not join America there immediately is a poor excuse for not proposing a hypersonic testing ban and calling the others to join. Indeed, if the United States is unwilling to forgo hypersonic testing for a time, others have every reason to be cynical about its real motives and intentions. I’m not sure that I understand those motivations. But I am reasonably certain that hypersonic missiles will not help to make America stronger or more secure, because it is clear, from their programs already in progress, that other nations will not allow the United States to claim a monopoly on hypersonic weaponry.

One repeated criticism of hypersonic weapons programs is that they are technology-driven rather than mission-driven, i.e., the United States and other major powers seem to be developing this technology just because they can. Indeed, every step in the history of the nuclear arms race, from the first fission bombs to thermonuclear weapons to intercontinental ballistic missiles and their multiple independently targetable reentry vehicles—one might even say the history of all weapons—has been technology-driven to a greater or lesser degree. Those programs that were driven more by a specific goal than by capability, such as the dream of rendering nuclear weapons impotent via missile defense, have largely failed. Today, a resurgent arms race is being driven by emerging technologies­—hypersonic weapons, space weapons, autonomous weapons controlled by artificial intelligence—and the question before humanity is whether in the 21st century, technology will be the driver of history, or humans will assert the sovereign right to determine their own future. I believe the cause of humanity is served by placing roadblocks in the path of a technology-driven arms race. A ban on the testing of hypersonic missiles presents one terribly important and relatively simple opportunity to do just that.